The present invention relates generally to multichannel communications receivers such as used in cable television (CATV) systems and, more particularly, to improvements for reducing third order intermodulation distortion in these types of receivers.
The number of channels characterizing CATV systems has greatly expanded over the years to a point where it is presently not uncommon to provide up to 50 or more different channels of programming for transmission to a subscriber's receiver. While the large number of channels provided by such a system affords the benefit of greatly increased programming variety, signal reception is somewhat degraded by a corresponding increase in distortion introduced at the signal receiver. In particular, it is well known that as the number of channels provided by a CATV system increases much beyond 12, the predominant distortion limiting performance changes from cross-modulation to third order intermodulation. In fact, it has been found that third order spurious signals that fall in the spectrum of interest increase exponentially as the number of channels increases.
Third order intermodulation distortion is produced by non-linear circuit components which develop spurious beat products that stack or accumulate in a communications receiver when the broadcast channels are evenly spaced (e.g. in a CATV system where the channels are spaced 6 MHz apart). This stacking or accumulation of third order intermodulation beat products, which is manifested in the reproduced video image in terms of increased background noise, is illustrated in FIGS. 1A-1C which represent the frequency spectra of a communications system characterized by two, three and four channels respectively. Referring initially to FIG. 1A, the two bold lines represent two carriers f.sub.1 and f.sub.2 transmitted over adjacent channels of the system. These two carriers, when processed by a receiver having non-linear circuit components, produce third order intermodulation distortion products 2f.sub.1 -f.sub.2 and 2f.sub.2 -f.sub.1 at the intercarrier spacing. In FIG. 1B, the bold lines represent three carriers f.sub.1, f.sub.2, and f.sub.3 transmitted over three equally spaced channels and the third order intermodulation products resulting therefrom. It will be noted that in addition to products of the form 2f.sub.x -f.sub.y (commonly referred to as two tone third order products) further products of the form f.sub.x +f.sub.y -f.sub.z are developed, the latter form of intermodulation product frequently being referred to as triple beat distortion. These third order intermodulation products are incoherent with the channel carriers superimposing a noisy background thereon. FIG. 1C represents a communication system wherein four carriers f.sub.1, f.sub.2, f.sub.3, f.sub.4, are transmitted over four equally spaced channels. It will be observed that the third order intermodulation products (of the form 2f.sub.x -f.sub.y and f.sub.x +f.sub.y -f.sub.z) begin to accumulate or stack about the carriers to further increase the distortion and background noise characterizing the system. As the number of channels is increased further, the stacking effect of the third order intermodulation products increases at an exponential rate to a point where the introduced distortion becomes quite objectionable.
Prior art attempts to reduce the effects of third order intermodulation distortion have included efforts to develop more linear receiver components, but these efforts have not yet proven satisfactory. Sometimes, an attempt is made to reduce third order intermodulation distortion by operating the channel receivers at reduced power levels. However, this is not always possible where the receiver is designed for an existing broadcasting medium.
It is accordingly a basic object of the present invention to provide an improved technique for reducing third order intermodulation distortion in a multichannel communications receiver.
It is a further object of the invention to provide a circuit for reducing the level of intermodulation and nonlinear products in a multichannel communications receiver without requiring a corresponding reduction in operating power levels.
It is yet a further object of the invention to provide a circuit of the foregoing type which may be implemented using a minimum number of circuit components and which is relatively inexpensive to manufacture.